A VCSEL is a semiconductor laser diode in which the laser oscillation and the optical emission occur in a direction normal to the p-n junction plane. VCSELs include a semiconductor layer of optically active material, such as gallium arsenide or indium phosphide sandwiched between highly reflective mirror stacks. Conventionally, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light which builds up in the resonate cavity formed by the mirror stacks sandwiching the active layer.
Lasing structures require optical confinement in the resonating cavity and carrier confinement in the active region to achieve efficient conversion of pumping electron-hole pairs into stimulated photons through population inversion. In order to reach the threshold for lasing, the total gain of a VCSEL must be equal to or greater than the total loss of the VCSEL. However, the compact nature of VCSELs typically limits the volume of gain media integrated into the device. Therefore, low-threshold, high-efficiency VCSEL operation typically requires a low loss index guiding mechanism and highly reflective low loss mirrors.